NESA Chemistry Chemical Synthesis and Design
15 sample questions with marking guides and sample answers · Avg. score: 54.6%
Sulfuric acid is manufactured by the Contact process, the steps of which are outlined below.
Step One: Molten sulfur is burned in air at approximately 1000 °C:
Step Two: The resulting sulfur dioxide is converted to sulfur trioxide as shown in the following equilibrium reaction. It is conducted at a temperature of about 450 °C with a catalyst at a pressure of between 100 and 200 kPa:
Step Three: The resulting sulfur trioxide is absorbed into sulfuric acid, producing oleum (). Water is added to the oleum, producing 18 mol L sulfuric acid:
Use your understanding of collision theory and chemical equilibrium to discuss the reaction conditions for Steps 1 and 2 of the Contact process, given that the aim is to produce the greatest yield in the shortest time. In your discussion, also address economic concerns where appropriate.
Reveal Answer
High temperature increases the average kinetic energy of the particles, which means that the particles collide more frequently. Also, more of these collisions will have energy higher than the activation energy, which means a greater proportion of collisions are successful, and the reaction rate increases. The vanadium catalyst increases the rate of the forward reaction, and also the rate of the reverse reaction to an equal extent, as it provides an alternative pathway with a lower activation energy. Therefore, a greater proportion of the particles will have sufficient energy to react when they collide. High pressure or concentration has more particles per unit volume and so there is a higher frequency of collisions, and the reaction rate increases. As Step 1 is a combustion reaction, it essentially goes to completion at the high temperature and does not require a catalyst or high pressure. For Step 2, high temperature, high pressure and a catalyst would favour a high rate.
For equilibrium, which is only considered for Step 2, high temperature favours the reverse reaction because it is endothermic, and this decreases the SO(g) yield, which is not desired. A low temperature decreases the rate of reaction, which is also not desired. A high pressure favours the forward reaction because there are a greater number of moles of gas reactants, increasing the SO(g) yield which is desired.
Economically, high pressures are costly and dangerous.
Therefore, for Step 2, a compromise is required between the high temperature for rate and the low temperature for yield. A compromise is also required between the cost of higher pressures and the pressure that allows a satisfactory yield and rate.
Rates
| Descriptor | Marks |
|---|---|
1 mark each for any of the following, up to a maximum of 6 marks: Explains that high temperature increases the average kinetic energy of the particles, which means that the particles collide more frequently; Explains that more of these collisions will have energy higher than the activation energy, which means a greater proportion of collisions are successful, and the reaction rate increases; Describes that the vanadium catalyst increases the rate of the forward reaction (and also the rate of the reverse reaction to an equal extent) as it provides an alternative pathway with a lower activation energy; States that a greater proportion of the particles will have sufficient energy to react when they collide; Explains that high pressure (concentration) has more particles per unit volume and so there is a higher frequency of collisions, and the reaction rate increases; Identifies that as Step 1 is a combustion reaction, it essentially goes to completion at the high temperature (and does not require a catalyst or high pressure); States that for Step 2, high temperature, high pressure and catalyst would favour high rate. | 6 |
Equilibrium
| Descriptor | Marks |
|---|---|
1 mark each for any of the following (only considered for Step 2), up to a maximum of 3 marks: Explains that high temperature favours the reverse reaction because it is endothermic, and this decreases the SO3(g) yield; States that a low temperature decreases the rate of reaction; Explains that a high pressure favours the forward reaction because there are a greater number of moles of gas reactants, increasing the SO3(g) yield. | 3 |
Economics
| Descriptor | Marks |
|---|---|
States that high pressures are costly (and dangerous). | 1 |
Compromise
| Descriptor | Marks |
|---|---|
1 mark each for any of the following, up to a maximum of 2 marks: Identifies that for Step 2, a compromise is required between the high temperature for rate and the low temperature for yield; Identifies that a compromise is also required between the cost of higher pressures and the pressure that allows a satisfactory yield and rate. | 2 |
Green chemistry principles include the design of chemical synthesis processes that
use renewable raw materials and minimise unwanted products.
use renewable raw materials and minimise unwanted reactants.
use non-renewable raw materials and minimise unwanted products.
use non-renewable raw materials and minimise unwanted reactants.
Reveal Answer
use renewable raw materials and minimise unwanted products.
Green chemistry principles explicitly advocate for the use of renewable feedstocks (Principle 7) and the prevention of waste by minimizing the formation of unwanted by-products (Principles 1 and 2).
use renewable raw materials and minimise unwanted reactants.
While using renewable materials is a core goal, the principles focus on minimizing waste (unwanted products) generated during the reaction, rather than minimizing reactants.
use non-renewable raw materials and minimise unwanted products.
Green chemistry promotes the transition to renewable raw materials (such as agricultural products) rather than relying on depleting non-renewable resources like petroleum.
use non-renewable raw materials and minimise unwanted reactants.
This option is incorrect because green chemistry aims to use renewable resources, and the primary objective regarding synthesis efficiency is the reduction of waste products.
During fermentation, yeast will produce other volatile polar compounds that have similar boiling points to ethanol, .
Which one of the following methods would be most suitable to separate these compounds from ?
solvent extraction
simple distillation
fractional distillation
solvent extraction and distillation
Reveal Answer
solvent extraction
Solvent extraction relies on differences in solubility, which would likely be ineffective here since both ethanol and the other compounds are polar and may dissolve in similar solvents.
simple distillation
Simple distillation is only effective for separating liquids with significantly different boiling points (typically a difference of at least 25°C).
fractional distillation
Fractional distillation is specifically designed to separate miscible volatile liquids that have very similar boiling points.
solvent extraction and distillation
Adding solvent extraction is unnecessary and less efficient, as fractional distillation alone is the standard and most effective method for separating liquids with similar boiling points.
The reaction to produce methanal, , is shown below.
The primary role of the catalyst in the production of is to increase the
speed of all particles.
number of collisions per unit time.
proportion of particles that react.
overall kinetic energy of the system.
Reveal Answer
speed of all particles.
A catalyst does not change the speed of the particles. Increasing the temperature of the system would increase particle speed.
number of collisions per unit time.
A catalyst does not increase the frequency of collisions. Factors like increased concentration, pressure, or temperature would increase the number of collisions per unit time.
proportion of particles that react.
A catalyst provides an alternative reaction pathway with a lower activation energy, meaning a greater proportion of particles have sufficient energy to react upon collision.
overall kinetic energy of the system.
The overall kinetic energy of the system is determined by its temperature, not by the presence of a catalyst.
The Ostwald process is used in the conversion of ammonia to nitric acid according to the equations below.
Equation 1:
Equation 2:
Equation 3:
The reaction in Equation 1 is carried out with a platinum-rhodium catalyst at approximately 850.0 °C and 1500 kPa. Using collision theory, account for these conditions.
Reveal Answer
The platinum-rhodium catalyst increases the rate of reaction by providing an alternative reaction pathway with a lower activation energy. This means a greater proportion of collisions are successful.
The high temperature of about 850.0 °C increases the kinetic energy of the reacting particles. The particles move faster, so collisions occur more frequently, and a greater proportion of collisions have energy greater than the activation energy. Therefore, more collisions are successful.
The high pressure of 1500 kPa forces the gas particles closer together. This reduces the space between particles and increases the frequency of collisions. As a result, the number of successful collisions per second increases, so the reaction rate increases.
Catalyst
| Descriptor | Marks |
|---|---|
Recognises that rate is increased as the platinum-rhodium catalyst provides an alternate reaction pathway with a lower activation energy | 1 |
Recognises that there are an increased proportion of reacting particles colliding with energy greater than the activation energy (resulting in an increased frequency of successful collisions) | 1 |
Temperature
| Descriptor | Marks |
|---|---|
Recognises that increased temperature increases the average kinetic energy of particles and they collide more frequently | 1 |
Recognises that the increased proportion of collisions will have energy higher than the activation energy (which has been lowered due to the catalyst) | 1 |
Recognises that a greater proportion of collisions are therefore successful (leading to an increased rate of reaction) | 1 |
Pressure
| Descriptor | Marks |
|---|---|
Recognises that increased pressure reduces the space between reacting particles | 1 |
Recognises that at high pressure particles collide more frequently | 1 |
Recognises that this leads to a higher frequency of successful collisions | 1 |
A nitric acid plant requires a production of 1095 tonnes of nitric acid by means of the Ostwald process each day. If the conversion of ammonia to nitric acid is 77.65% efficient, calculate the volume of ammonia at standard temperature and pressure (STP) that must be fed into the process each day. Give your answer to the appropriate number of significant figures.
Reveal Answer
| Descriptor | Marks |
|---|---|
| 1 | |
| 1 | |
| 1 | |
| 1 | |
| 1 | |
Answer to 4 significant figures | 1 |
Ibuprofen is manufactured using two different processes.
| Process | Number of reagents used | Reagents | Reagents | Ibuprofen | Ibuprofen | Waste products | Waste products |
|---|---|---|---|---|---|---|---|
| Atoms | Atoms | Atoms | |||||
| 1 | 7 | 514.5 | 206.0 | 308.5 | |||
| 2 | 4 | 266.0 | 206.0 | 60.0 |
Calculate the atom economy for each process and draw conclusions about the economic and environmental impact of each process.
Reveal Answer
Process 1: atom economy =
Process 2: atom economy =
Process 2 has 37.4% better atom economy than process 1
Economic impact: Process 2 has a better atom economy than process 1 (fewer reagents are required).
Environmental impact: Process 2 is greener than process 1 because fewer waste products (atoms) are produced.
| Descriptor | Marks |
|---|---|
Calculates atom economy for Process 1 as 40% | 1 |
Calculates atom economy for Process 2 as 77% | 1 |
Concludes process 2 is cheaper as fewer reagent atoms are required | 1 |
Concludes process 2 is greener as fewer waste atoms are produced | 1 |
Calculate the percentage yield of magnesium ethanoate when 8.0 moles of ethanoic acid reacts with 6.0 moles of magnesium carbonate, producing 3.5 moles of magnesium ethanoate as shown in the equation.
44%
58%
88%
100%
Reveal Answer
44%
This value is obtained by dividing the actual yield (3.5 mol) by the initial moles of ethanoic acid (8.0 mol), neglecting the 2:1 stoichiometric ratio which dictates the theoretical yield is half the moles of the acid.
58%
This calculation incorrectly identifies magnesium carbonate as the limiting reactant. Since magnesium carbonate is in excess, the theoretical yield should be based on ethanoic acid.
88%
Ethanoic acid is the limiting reactant ( theoretical yield). The percentage yield is calculated as , which rounds to 88%.
100%
This option implies the actual yield equals the theoretical yield, but 3.5 moles is less than the calculated theoretical yield of 4.0 moles.
The circular economy of a bioethanol production process could be improved by
increasing the fermentation temperature to accelerate the reaction rate.
using a new strain of yeast to produce a higher yield of ethanol from glucose.
developing a separate process that converts waste carbon dioxide into a useful product.
carrying out advanced distillation techniques to reduce energy consumption.
Reveal Answer
increasing the fermentation temperature to accelerate the reaction rate.
Increasing the fermentation temperature improves reaction kinetics but does not address waste reduction or resource recycling, which are the core principles of a circular economy.
using a new strain of yeast to produce a higher yield of ethanol from glucose.
While a higher yield improves process efficiency, it does not involve repurposing waste streams or closing material loops characteristic of a circular economy.
developing a separate process that converts waste carbon dioxide into a useful product.
A circular economy focuses on eliminating waste by repurposing it; converting waste carbon dioxide into a useful product perfectly aligns with this principle by closing the material loop.
carrying out advanced distillation techniques to reduce energy consumption.
Reducing energy consumption improves the environmental footprint and sustainability of the process, but it does not represent the material recycling or waste valorization central to a circular economy.
Which one of the following reactions has the highest atom economy in the production of an organic molecule?
complete combustion of propyne,
reaction of iodine, , with propane,
reaction of bromine, , and propene,
formation of a dipeptide from alanine,
Reveal Answer
complete combustion of propyne,
Complete combustion produces carbon dioxide and water, which are inorganic molecules, and it involves fragmentation rather than forming a single desired product, resulting in a low atom economy.
reaction of iodine, , with propane,
The reaction of an alkane with a halogen is a substitution reaction that produces a hydrogen halide () as a byproduct, meaning the atom economy is less than 100%.
reaction of bromine, , and propene,
The reaction of an alkene with a halogen is an addition reaction. Because all reactant atoms are incorporated into the single desired organic product (1,2-dibromopropane), it has an atom economy of 100%.
formation of a dipeptide from alanine,
The formation of a dipeptide from amino acids is a condensation reaction that produces water as a byproduct, which reduces the atom economy below 100%.
Which of the following is least likely to be a characteristic of a process classified as green chemistry?
is energy intensive
utilises renewable feedstocks
produces less waste
utilises fewer toxic solvents
Reveal Answer
is energy intensive
Green chemistry principles advocate for energy efficiency, so an energy-intensive process contradicts these goals and is least likely to be a characteristic.
utilises renewable feedstocks
Using renewable feedstocks is a core principle of green chemistry, making it highly likely to be a characteristic of such a process.
produces less waste
Waste prevention is the first principle of green chemistry, so producing less waste is a key characteristic, not the least likely.
utilises fewer toxic solvents
Minimizing the use of toxic solvents is a fundamental goal of green chemistry to reduce environmental and health hazards.
Aspirin (CHO) can be produced from a reaction between salicylic acid (CHO) and acetic anhydride (CHO) with ethanoic acid being a minor product.
Calculate the mass of salicylic acid required to produce 8.25 g of aspirin if the percentage yield of the reaction is 60%. Show your working.
Reveal Answer
1:1 Aspirin : salicylic acid (SA)
| Descriptor | Marks |
|---|---|
determines molar mass aspirin is 180.17 | 1 |
determines moles aspirin | 1 |
determines moles salicylic acid | 1 |
calculates mass salicylic acid | 1 |
Vanillin is a widely used ingredient in the food industry. Pure vanillin has a melting point of 82–83 °C.
To reduce costs, it is often blended with significant amounts of cheaper compounds that have similar melting points.
A blended sample was tested.
The melting point range of the blend would be closest to
68–79 °C
79–80 °C
82–83 °C
100–109 °C
Reveal Answer
68–79 °C
Blending vanillin with other compounds creates an impure mixture, which disrupts the crystal lattice. This results in a melting point depression, causing the melting point to be significantly lower and the range to be broader.
79–80 °C
While the melting point is lower, a blend with significant amounts of impurities would result in a much broader melting point range than just 1 °C.
82–83 °C
This is the melting point of pure vanillin. An impure blend would not maintain the same sharp, high melting point as the pure substance.
100–109 °C
Impurities typically depress (lower) the melting point of a substance rather than elevate it.
Which option is a principle of green chemistry?
avoid chemical derivatives
decrease energy efficiency
prevent catalytic reactions
minimise atom economy
Reveal Answer
avoid chemical derivatives
Reducing or avoiding unnecessary chemical derivatives (such as blocking groups or protection/deprotection steps) is one of the 12 principles because these steps consume additional reagents and generate waste.
decrease energy efficiency
Green chemistry aims to increase energy efficiency, not decrease it, by designing synthetic methods to operate at ambient temperature and pressure whenever possible.
prevent catalytic reactions
The principles advocate for the use of catalytic reagents rather than preventing them, as catalysts are superior to stoichiometric reagents due to their reusability and selectivity.
minimise atom economy
The goal is to maximize atom economy, ensuring that the largest possible percentage of starting materials ends up in the final product to minimize waste.
Which set of conditions below would optimise the yield of methanol in the following industrial process?
low pressure, high temperature
high pressure, high temperature, catalyst
low pressure, low temperature, catalyst
high pressure, low temperature
Reveal Answer
low pressure, high temperature
Low pressure favors the side with more moles of gas (reactants), and high temperature favors the endothermic reverse reaction, both of which decrease the yield of methanol.
high pressure, high temperature, catalyst
While high pressure increases yield, high temperature favors the endothermic reverse reaction, decreasing the yield. Additionally, a catalyst only speeds up the reaction and does not affect the equilibrium yield.
low pressure, low temperature, catalyst
While low temperature increases yield, low pressure favors the side with more moles of gas (reactants), decreasing the yield.
high pressure, low temperature
High pressure favors the product side which has fewer moles of gas (1 mole vs 3 moles), and low temperature favors the exothermic forward reaction, both of which maximize the yield of methanol according to Le Chatelier's principle.
A key advantage of using renewable feedstocks in the manufacture of organic compounds is that they
eliminate any associated environmental impact.
reduce reliance on finite natural resources.
lead to the production of fewer by-products.
reduce the energy requirements of the manufacturing process.
Reveal Answer
eliminate any associated environmental impact.
While renewable feedstocks can lower certain environmental impacts like net carbon emissions, they do not completely eliminate them, as their harvesting and processing still require land, water, and energy.
reduce reliance on finite natural resources.
Renewable feedstocks, such as plant biomass, can be continuously replenished naturally, which directly decreases our dependence on depleting, finite resources like petroleum and coal.
lead to the production of fewer by-products.
The generation of by-products is determined by the specific chemical reaction pathway and the atom economy of the process, not inherently by whether the starting materials are renewable.
reduce the energy requirements of the manufacturing process.
Converting renewable feedstocks into usable organic compounds can actually be highly energy-intensive and does not inherently guarantee a reduction in manufacturing energy requirements compared to traditional petrochemical methods.